The induced flow through a lifting rotor in forward flight

Abstract

A technique has been developed to enable the measurement of the velocity field in the vicinity of a lifting rotor. This involved the use of a triaxial hot-wire probe coupled to a mini-computer. Using this technique extensive flow measurements in one plane (z/R = -0.075) under the rotor were made at advance ratios of 0.1 and 0.067. In addition the rotor thrust, torque and blade bending moments were also measured. The results of the analysis of the time averaged flow show clear evidence that the wake of the rotor rapidly rolls up into 2 'fixed-wing' type vortices. Also indicated is that the value of K in Glauert's induced velocity equation is underestimated by about 50%. It is also shown that results obtained from tests carried out at the same advance ratio, but different tip speeds, do not produce the same non-dimensionalised velocities at all positions. Analysis of the time varying results has shown that the roll-up of the tip vortex is completed within approximately 1 chord length of the blade and that the strength of the tip vortex is equal to the maximum value of the blade bound vorticity. The time varying data also reveals the influence of the turbulent blade wake and this has been used to obtain estimates of the blade profile drag. It is believed that this is the first time this has been observed in helicopter wake measurements and is potentially of great use in investigating the reductions in drag from using advanced blade sections and tip planforms. Aft of the rotor velocity traces showing the intersection of the tip vortex by the probe has been analysed to provide estimates of the vortex core size. These results also show evidence of axial flow in the vortex core.

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